Optical fiber lasers and amplifiers are known in the art. In such lasers and amplifiers, rare earth materials disposed in the core of the optical fiber laser or amplifier receive pump radiation of a predetermined wavelength and, responsive thereto, provide or amplify light of a different wavelength for propagation in the core. For example, the well known erbium doped fiber amplifier (EDFA) receives pump radiation having a wavelength of 980 or 1480 nanometers (nm) and amplifies an optical signal propagating in the core at a wavelength in the 1550 nm region.
In such optical fiber lasers and amplifiers, the pump radiation can be introduced directly to the core, which can be difficult due to the small size of the core, or can be introduced to the cladding surrounding the core and absorbed by the core as the rays propagating in the cladding intersect the core. Lasers and amplifiers with the pump radiation introduced to the cladding are known as “cladding-pumped” optical devices, and facilitate the scale-up of lasers and amplifiers to higher power systems.
Absorption per unit length is a useful figure of merit for evaluating a cladding-pumped optical fiber laser or amplifier. It is typically desirable that the amplifier or laser have a high absorption per unit length, indicating that the pump radiation frequently intersects the core. Unfortunately, when the cladding has a circular outer circumference, the pump radiation can essentially propagate down the optical fiber while spiraling around the core without substantially intersecting the core. This leads to a low absorption per unit length of the optical fiber device, and hence detracts from the performance of the optical fiber laser or amplifier.
Various approaches are known in the art for enhancing the intersection of the pump radiation with the core and hence raising the absorption per unit length of the optical fiber amplifier or laser. For example, as disclosed in U.S. Pat. No. 4,815,079, issued Mar. 21, 1989 to Snitzer et al., the core can be offset from the center of the optical fiber so as to enhance the intersection of pump light with the core. In another approach, the inner cladding has a “D”-shaped outer circumference that includes a flat section, as disclosed in U.S. Pat. No. 5,864,645, issued Jan. 26, 1999 to Zellmer et al. In another prior art optical fiber, the outer circumference of the cladding is shaped as a polygon, such as a diamond, as disclosed in U.S. Pat. No. 5,533,163, issued Jul. 2, 1996 to Muendel. Other approaches include providing a star-shaped outer circumference of the cladding, as disclosed in U.S. Pat. No. 5,949,941, issued Sep. 7, 1999 to DiGiovanni. See also WO 99/30391, published Jun. 17, 1999, disclosing an optical fiber having a core, inner and outer claddings, and a series of perturbations or irregularities formed in the otherwise circular outer boundary of the inner cladding. The optical fiber is drawn from a preform having rods inserted into holes drilled into the preform for producing the irregularities.
In the foregoing prior art fibers, the non-circular shape of the outer circumference is understood to cause ray distortion and mode mixing of light, thereby directing the light rays of the cladding radiation to the core, and avoiding trapping light in spiral paths that do not intersect the core.
The designs discussed above can have disadvantages. For example, a fiber having an offset core can be difficult to interconnect with other optical components. Designs, such as the diamond and polygon designs discussed above, that require the circumference of the cladding to predominately consist of flat areas, can be difficult to fabricate. The flat areas, which are typically first machined into the preform from which the optical fiber is drawn, tend to deform and change shape when the fiber is drawn at the most desirable temperatures. Accordingly, often the draw temperature is reduced to preserve the desired shape of the outer circumference of the cladding. A reduced draw temperature typically produces optical fibers having higher attenuation and lower mechanical strength. In addition, the star shaped configuration disclosed in U.S. Pat. No. 5,949,941 can be difficult to manufacture. Accordingly, an improved cladding-pumped optical device and/or techniques for manufacturing such optical fiber devices would be a welcome advance in the art.
It is desirable to address one or more of the foregoing disadvantages and drawbacks of the prior art.